Build a push server (self-server), GCM (FCM) well-known! Can you tell me a little bit about myself?

I have built a chat server through a web socket once

This time, I'd like to send a message or push notification to clients by announcing it on the server. = Build a push server

You can use gcm and fcm, but I'd like to think of my own server.

First of all, I don't have enough knowledge yet, but personally, when I communicate through a web socket, the server is waiting and sends a message immediately when the client requests it I think it's a continuous connection. For example, even on mobile, you have to be ready to receive it in the background to spray it on the screen.

1. Then doesn't the battery run out a lot? What kind of push method do you use?

ex) Polling & Long Polling ,  Streaming  등 상세하게 부탁드려요

2. So how do you generally build push servers? (Self-servers) So how do Google's GCM and FCM use push?

The push service I'm trying to build is that the client doesn't have to request anything from the server, but rather, I want to display notifications like broadcasting that was sprayed on the client's screen when the server sent a message.

If you know about push server, please give me some advice!